1. Field of the Invention
This invention relates to thin-film transistor elements and methods of making the same. More particularly, it relates to thin-film transistor element structures used in active matrix type liquid crystal displays, and methods of making the same.
2. Description of the Related Art
In recent years, active matrix type liquid crystal displays in which thin-film transistors (TFTs) using a hydrogenated amorphous silicon film are utilized as switching elements for display picture elements are being produced in large quantities. Especially with the popularization of notebook type personal computers, the demand for liquid crystal displays is growing rapidly and, therefore, an improvement in their productivity is being needed.
Referring to FIG. 17, there is shown a sectional view of an inverted staggered type thin-layer transistor element which is commonly used as a switching element for each picture element of a liquid crystal display. First of all, a metal for use as a gate electrode is deposited on a transparent insulating substrate 10, and patterned into a desired shape to form a gate electrode 11. Then, a silicon nitride film 12 serving as a gate insulating film, an amorphous silicon film 13, and an n-doped amorphous silicon film 14 for making ohmic contacts of source-drain regions are successively formed thereon, and n-doped amorphous silicon film 14 and amorphous silicon film 13 are patterned into an island-like shape. Subsequently, a metal for use as source-drain electrodes is deposited and patterned into desired shapes to form source-drain electrodes 15. Finally, the undesired n-doped amorphous silicon film 14 above the channel is etched together with a part of amorphous silicon film 13 in consideration of a margin. Thus, a thin-film transistor element as illustrated in FIG. 17 is completed.
As an inverted staggered type thin-film transistor element of this type, Japanese Patent Publication No. 51069/""92 has proposed a thin-film transistor in which an n-doped amorphous silicon film is formed so as to cover the whole surface of the island-like amorphous silicon film and so as to impart an off resistance of not less than 109 xcexa9 to the thin-film transistor (i.e., the n-doped amorphous silicon film above the channel is not removed). However, in order to obtain good ohmic contact characteristics, the n-doped amorphous silicon film must have a resistivity of not greater than 104 xcexa9cm. Moreover, in order to achieve an off resistance of not less than 109 xcexa9 for a typical thin-film transistor element size as represented by a (channel width)/(channel length) ratio of 10, the thickness of the n-doped amorphous silicon film must be limited to 10 nm or less even if the resistivity thereof is not greater than 104 xcexa9cm. Where it is desired to achieve a satisfactory panel representation by using thin-film transistor elements as driving elements for the picture elements of a liquid crystal display, an off resistance of about 1010 to 1011 xcexa9 is actually required. In order to obtain such an off resistance, the n-doped amorphous silicon film must have a thickness of about 0.1 to 1 nm. However, such a very thin n-doped amorphous silicon film involved a problem in that it fails to give good ohmic contact characteristics and hence causes a marked reduction in on-state current.
Moreover, in recent years, technical developments are being made to improve the aperture ratio of each picture element section of a liquid crystal display by using, as protective insulating films, insulating films formed by applying and thermally curing various polymeric materials. In these applied insulating films, a film thickness of about 2 to 3 xcexcm can be easily obtained, and their relative permittivities are equal to about xc2xd of that of a conventionally used silicon nitride film. Accordingly, even if the transparent conductive picture element electrodes formed on such an applied insulating film overlap with the data lines and the signal lines, the coupling capacity due to such overlap is very low and, therefore, display defects such as crosstalk are minimized. Thus, it becomes possible to achieve a high aperture ratio while maintaining satisfactory display characteristics. In this respect, an explanation is given below with reference to FIG. 18.
A metal for use as a gate electrode is deposited on a transparent insulating substrate 10, and patterned into a desired shape to form a gate electrode 11. Then, a silicon nitride film 12 serving as a gate insulating film, an amorphous silicon film 13, and an n-doped amorphous silicon film 14 for making ohmic contacts of source-drain regions are successively formed thereon, and n-doped amorphous silicon film 14 and amorphous silicon film 13 are patterned into a desired island-like shape. Subsequently, a metal for use as source-drain electrodes is deposited and patterned into desired shapes to form source-drain electrodes 15. Moreover, the undesired n-doped amorphous silicon film 14 above the channel is etched together with a part of amorphous silicon film 13 in consideration of a margin. Thereafter, a protective insulating film (or applied insulating film) 18 is formed over the whole surface. Finally, a transparent conductive picture element electrode 19 is formed so that it is electrically connected to the source electrode through a contact hole. Thus, a thin-film transistor element is completed. This technique has been reported, for example, by Y. Takafuji et al. [SID ""93, Digest, p. 383 (1993)] and Jeong Hyun Kim et al. [AM-LCD 96, Digest, p. 149 (1996)].
At present, in order to reduce the prices of liquid crystal displays, it is strongly desired to simplify the thin-film transistor fabrication process and achieve an improvement in the throughput thereof. In particular, inverted staggered type thin-film transistor elements as described above are most widely utilized in liquid crystal displays because of their excellent element characteristics and stability, and a simplification of their fabrication process and an improvement in the throughput thereof are believed to contribute greatly to a reduction in the prices of liquid crystal displays. As described above, in the case of conventional inverted staggered type thin-film transistor elements, it is necessary to etch the undesired n-doped amorphous silicon film above the channel during the fabrication process thereof. To this end, it is difficult to etch the n-doped amorphous silicon film selectively at a high selectivity ratio to the underlying amorphous silicon film. Accordingly, the n-doped amorphous silicon film has been etched together with a part of the underlying amorphous silicon film in consideration of a margin.
However, the amorphous silicon film surface, i.e., back channel interface, having been exposed to an etching gas is severely affected by process damage and hence has a very high surface state density due to defects. Consequently, if the thickness of the amorphous silicon film in the channel region is decreased to about 150 nm or less after etching, the on-state characteristics of the thin-film transistor element will be significantly reduced under the influence of the surface state on the back channel side. For theses reasons, it has been necessary to form an amorphous silicon film having a thickness of as large as about 300 nm.
As described above, conventional inverted staggered type thin-film transistor elements have involved the following two important problems:
(1) It is necessary to etch the undesired n-doped amorphous silicon film above the channel together with a part of the underlying amorphous silicon film in consideration of a margin.
(2) In order to obtain good on-state characteristics, the amorphous silicon film must be thick.
It is believed that these problems raise the costs of liquid crystal displays for the following reasons:
As to the above problem (1), there is only a slight difference in etching selectivity between the n-doped amorphous silicon film and the amorphous silicon film, so that the amount of etching tends to vary in the panel. Consequently, the on-state characteristics of thin-film transistor elements are reduced in the regions where the amount of etching is greater (i.e., the regions where the amorphous silicon film constituting the channel has become thinner after etching). This causes an image to be unevenly displayed on the panel, resulting in a low yield of products.
As to the above problem (2), the plasma CVD process for forming an amorphous silicon film and the dry etching process for patterning it into an island-like shape cause a reduction in throughput and hence a rise in cost. Moreover, if the amorphous silicon film having high photosensitivity is thick, the off-state photocurrent of the thin-film transistor element will be increased, resulting in a reduction in holding characteristics. This may also cause an image to be unevenly displayed on the panel.
For these reasons, it is needed to develop, for use with inverted staggered type thin-film transistor elements, a device technique which makes it unnecessary to etch the undesired n-doped amorphous silicon above the channel and permits the amorphous silicon film to be made thinner.
Furthermore, the convention structure for achieving a high aperture ratio has involved a problem concerning the long-term reliability and stability of transistor characteristics in that, since the amorphous silicon film constituting the active layer of the thin-film transistor and the applied insulating film come into direct contact at the back channel interface, moisture and mobile ions present in the applied insulating film (their contents are generally much higher than those in a silicon nitride film formed by a plasma CVD process) affect the back channel characteristics of the transistor. This may also cause an image to be unevenly displayed on the liquid crystal display. In order to minimize this problem, it has been conventional practice to use a channel protection type thin-film transistor element (having an inverted staggered type structure in which the back channel interface is coated with a silicon nitride film in advance), or stabilize the back channel interface characteristics by forming a silicon nitride film on the back channel interface of the amorphous silicon film from which the n-doped amorphous silicon film has been removed, as a pretreatment step preceding the formation of an applied insulating film, and thereafter forming the applied insulating film thereon. However, these measures increase the number of plasma CVD film-forming steps and patterning steps and hence causes a rise in cost.
It is an object of the present invention to provide a device technique which makes it possible to make thin-film transistor elements having stable characteristics even if an applied insulating film is used and to achieve a high aperture ratio in liquid crystal displays.
In order to solve the above-described problems, the present invention provides:
an inverted staggered type thin-film transistor element comprising a transparent insulting substrate having thereon at least a gate electrode, a gate insulating film, an island-like amorphous silicon film, source-drain electrodes, and an n-doped amorphous silicon film formed as an intermediate layer in the regions where the island-like amorphous silicon film overlaps with the source-drain electrodes, wherein the thin-film transistor element has an insulating film obtained by forming an n-doped amorphous silicon film tentatively in the region where the island-like amorphous silicon film does not overlap with the source-drain electrodes, and modifying the so-formed n-doped amorphous silicon film by a plasma treatment; and
an inverted staggered type thin-film transistor element comprising a transparent insulting substrate having thereon at least a gate electrode, a gate insulating film, an island-like amorphous silicon film, source-drain electrodes, and an n-doped amorphous silicon film formed as an intermediate layer in the regions where the island-like amorphous silicon film overlaps with the source-drain electrodes, wherein the thin-film transistor element is made by forming an n-doped amorphous silicon film tentatively in the region where the island-like amorphous silicon film does not overlap with the source-drain electrodes, modifying the so-formed n-doped amorphous silicon film into an insulating film by a plasma treatment, and removing the resulting insulating film with the aid of a solution containing hydrofluoric acid.
By applying the present invention to inverted staggered type thin-film transistor elements, it becomes unnecessary to etch the undesired n-doped amorphous silicon film above the channel together with a part of the underlying amorphous silicon film in consideration of a margin, and it becomes possible to make the amorphous silicon film thinner while maintaining excellent characteristics.
Especially when the n-doped amorphous silicon film is not etched but modified into an insulating film by a plasma treatment, a good and stable back channel interface having a low defect density can be created. Moreover, the resulting insulating film can readily be removed with the aid of a solution containing hydrofluoric acid. This is effective in terminating the dangling bonds of silicon present at the back channel interface with hydrogen atom in hydrofluoric acid and thereby achieving a further reduction in defect density.
Furthermore, the use of thin-film transistor elements in accordance with the present invention make it possible to achieve a high aperture ratio in liquid crystal displays while maintaining stabler characteristics than those of conventional thin-film transistor elements, and without increasing the number of process steps. The reasons for this are that an applied insulating film formed from a polymeric material and having a low relative permittivity may be used as a protective insulating film and picture element electrodes may be formed thereon so as to overlap with the signal lines and the data lines, and that an insulating film formed by modification based on a plasma treatment is present between the applied insulating film and the amorphous silicon film and this functions as a protective film for securing the stability of thin-film transistor element characteristics. Especially when a thermosetting resin is used as the polymeric material for the formation of an applied insulating film, a marked reduction in cost can be expected for such reasons as a low material cost.
Thus, the present invention makes it possible to achieve a high aperture ratio in liquid crystal displays while securing desirable thin-film transistor elements characteristics and, moreover, to reduce the manufacturing costs of such high-performance liquid crystal displays.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.